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The kidneys are bean shaped, each a little smaller than the size of a fist, and lie in the upper back, just below the ribcage. They filter the blood to remove waste products and control the amount of water in the body.

It is not clear why there are normally two kidneys. The human body does not need two kidneys, it could manage perfectly well with one kidney. It may just be that it has been useful for us to develop some parts of the body in pairs (arms and legs), so other parts doubled up as well. It is also possible that there is an evolutionary advantage in having a spare kidney, and this is certainly important in modern medicine, because people can live normal lives with one kidney.


If one kidney is small and the other one is completely normal, it is usually possible to lead a normal life without it affecting health.  Having one small kidney can increase the chance of developing some health problems such as high blood pressure, even if the other kidney is normal.

However, if both kidneys are small, this can be more of a concern and if there is not enough healthy kidney tissue, kidney failure can develop. Therefore people who are found to have a small kidney normally have some medical tests performed to see whether further treatment or observation over a period of time is necessary.


Many people lead a completely normal life with a single kidney. There are, however, many causes of a single kidney, and the outlook depends on the cause and on the condition of the single kidney. The causes are listed below.


The commonest causes of a small kidney are:-

  1. The kidney is small at birth, or never grows with the rest of the body. This is a common cause of a small kidney detected in childhood. The medical name for this is congenital dysplasia. In some people both kidneys can be affected by congenital dysplasia, and there may be kidney failure. The small kidney may be in a normal position in the upper back, or may have failed to move up from the lower abdomen before birth. A kidney in the lower abdomen is called a pelvic kidney. Many pelvic kidneys have normal function, but in some people a pelvic kidney can be small or have abnormalities in the drainage system, giving a risk of recurrent infection.
  2. The kidney is damaged because of a faulty drainage system, usually a condition called reflux nephropathy. This is a common cause of a small kidney detected in childhood or in a young adult. Reflux nephropathy can affect both kidneys. More information on reflux nephropathy is available from the NKF.
  3. An infection in a kidney can cause it to shrink. Normally kidney infections do not cause permanent damage to a kidney, or leave a small scarred area in the kidney. Occasionally, though, a severe kidney infection (acute pyelonephritis) can damage the kidney so much that it becomes small. An infection bad enough to cause this may occur in someone with reflux nephropathy (see the paragraph above).
  4. The kidney is starved of blood by a narrowing in the artery supplying the kidney with blood. This condition is a common cause of a small kidney in older people, especially if there is also a history of angina or heart attack, or narrowed arteries to the legs. Click here for more information on renal artery stenosis.
  5. Many other diseases which damage the kidney, such as glomerulonephritis Glomerulonephritis for more details about glomerulonephritis), can cause a kidney to shrink and become small, but glomerulonephritis usually affects both kidneys equally, and this section of medical information is really concerned with conditions where one kidney is smaller than the other.


  1. One kidney has been removed surgically, after an accident or because of a problem such as cancer or extensive kidney stones. So long as the other kidney is normal, people usually lead a normal life.
  2. Two kidneys joined together. In the womb, before birth, the kidneys form in the lower abdomen and move up. If the two immature kidneys join together, they cannot move all the way up the abdomen, and get stuck in the middle, in a lower than normal position. This large single kidney is called a horseshoe kidney because of its shape. A horseshoe kidney should function as normal with no problems. However, if any kidney disease such as a tumour or stones develop in a horseshoe kidney, treatment is much more difficult than if there are two kidneys. Problems with the drainage system are more common in horseshoe kidneys, and these may cause infections in some people with horseshoe kidney.
  3. If one kidney is small, for any of the reasons given above, it may be so hard to see on X-rays and scans that doctors tell someone they have a single kidney, or single functioning kidney, even though, strictly speaking, both kidneys are present.


Tests are performed to determine whether the better kidney is completely normal, and also to determine the cause of the small or missing kidney. The exact tests needed depend upon the details of each individual case. Some of the following will be performed:-

  1. Blood tests to measure the overall level of kidney function.
  2. Urine tests to see if infection is present.
  3. Blood pressure measurement, as high blood pressure is very common in kidney diseases.
  4. Scans and X-rays of the kidneys. The exact tests depend on the details of each case, but an ultrasound (sound wave) scan is usually performed, and this may be followed by measurement of the level of function and efficiency of drainage in each kidney (radio-isotope scan), or a magnetic resonance or CT scan to look at the arteries, or an X-ray with dye in the bladder to look for reflux nephropathy (called a micturating cystogram).


Many people with a small or single kidney have excellent health with no problems. The chances of complications or problems developing depend on the cause and severity in each individual. The specialist should advise each individual about problems that could develop in their case. Some of the problems that can be seen are:-

  1. Pain in the area of the kidney of there is infection, or pain passing urine (cystitis).
  2. High blood pressure.
  3. Kidney failure if the better kidney is also diseased. 


Many people need no treatment.

If there are urine infections, a high daily fluid intake may reduce the frequency of infection. In women, emptying the bladder after sexual intercourse may reduce infection rates. Cranberry juice, drunk daily, has been shown to reduce urine infections in some people with recurrent infections. Urine infection is also treated with antibiotics, and in some people with recurrent infections, long courses of antibiotics may be used. Lifelong treatment is sometimes used. Long term antibiotics may encourage the development of bugs resistant to the antibiotic, but either by switching to a different class of antibiotic when an infection occurs, or by using a different preventative antibiotic on a monthly cycle, resistance problems can be reduced.

High blood pressure, if it develops, usually requires drug treatment. Losing weight and reducing salt and alcohol intake may reduce the blood pressure.

Painkillers may be needed if a kidney is painful. Paracetamol can be used safely in people with kidney diseases, but do not exceed the maximum recommended dose. Ibuprofen (on sale in the UK as ‘Nurofen’ or ‘Advil’) is an effective painkiller but may not be suitable for most people with kidney trouble, and a doctor should be consulted before the use of ibuprofen or other non-steroidal anti-inflammatory pain killers. Other painkillers can be used, as recommended by a doctor.


Surgical removal of a small kidney is not often performed, but sometimes it is necessary. If a small kidney is causing no problems there is no need to remove it. If the kidney is causing pain or recurrent infection, or is suspected to be a cause of high blood pressure, removal may be indicated. Doctors are usually reluctant to remove a kidney that is doing useful work, even if it is causing some trouble, in case a problem develops with the better kidney in the future. If a small kidney is providing more than 25% (one quarter) of the total level of kidney function (this can be measured using a test called a radioisotope scan), doctors often suggest trying to control any problems caused by the kidney with drugs (such as long term antibiotics for infection), before removing the kidney. 


If the cause of a small kidney is reflux nephropathy or congenital dysplasia (the meaning of these terms is described above), some checks in the family should be performed. Although doctors recognise that these conditions can run in families, the rules of inheritance are not precise, and there are at present no genetic blood tests or DNA tests that can be used to check for the conditions. Therefore family screening consists of checking the kidneys by ultrasound scan (a simple test in the X-ray department).

The exact recommendations vary slightly from centre to centre, and are changing quickly as the understanding of kidney diseases improves. At present, the author suggests that if someone has a small kidney from reflux nephropathy or congenital dysplasia, that their children have an ultrasound scan to check the sizes and shapes of the kidneys. If there are any abnormalities, a specialist should be consulted. If the child of someone with reflux nephropathy has a normal scan but has repeated urine infections, a specialist should also be seen. The parents and brothers and sisters of the affected person can also be checked, if they wish, with an ultrasound scan of the kidneys. If a brother or sister has a normal scan, their children need not be checked unless they have recurrent urine infections, or there is a strong family history of kidney failure.


So long as the other kidney is normal and there are none of the complications listed above, life should be normal. It will probably be necessary to mention the problem when applying for life insurance or illness insurance, and an insurance company may ask for a specialist report. Women should usually be able to have a normal pregnancy, though problems such as urine infection or high blood pressure may appear for the first time during pregnancy. Doctors and midwives will undertake careful observation during pregnancy in someone known to have kidney problems. 

Last reviewed September 2022
Next review September 2025

Reviewed by Dr Oshini Shivakumar 'Specialist Renal Registrar'

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The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.